Papillomaviruses (PVs) infect the epithelia of animals and man where they generally induce benign proliferation at the site of infection. However, there is a strong association between malignant progression of human genital lesions and certain HPV types, most frequently HPV16. We have generated virus-like particles (VLPs) for HPV16 and other PVs that consist of the L1 major capsid protein or L1 plus L2, the minor capsid protein. To explore the possibility that these reagents could potentially serve as the basis for a subunit vaccine to prevent PV infection, we have conducted prophylactic vaccine trials in two animal models. After systemic inoculation, cottontail rabbit (CR)PV VLPs protected rabbits from cutaneous challenge with infectious CRPV and bovine (B)PV type 4 VLPs protected cattle from oral mucosal challenge with infectious BPV4. Protection could be passively transferred with serum. We have developed two assays that should be useful for quantitatively evaluating neutralizing antibody titers in human VLP- based vaccine trials: an in vitro neutralization assay for HPV16 and a VLP hemagglutination inhibition assay. In serological studies using an HPV VLP-based ELISA, we have found no evidence of HPV16 infection in young children or teenage girls prior to the initiation of sexual activity. These results support the conclusion that the predominant mode of HPV16 transmission is venereal and therefore that a prophylactic We have determined that the virions of two geographically distinct and genetically divergent HPV16 variants are serologically cross reactive. These results suggest that vaccination with the VLPs of a single HPV16 variant might afford protection worldwide. In a series of studies to quantitatively assess the risk associated with HPV16 infection for various human cancers, we and our collaborators have found a strong association between the presence of HPV16 virion antibodies and cervical, vulvar, and anal, but not penile cancer, in retrospective case-control studies. In prospective studies, seropositivity was strongly associated with the subsequent development of cervical and esophageal, but not laryngeal or oral cancer.